1. Field of the Invention
The present invention relates to a chip bead element employed in electronic circuits, and more specifically it relates to a chip bead element that demonstrates noise absorption characteristics over a broad range at a high frequency in the GHz band.
2. Discussion of Background
As electronic devices become ever thinner and smaller, circuit patterning on the internal board has come to be designed to achieve a higher density and electronic components that are to be mounted are now mounted at an extremely high density. In addition, with higher frequencies and digitalization achieved for signals in recent years, noise increasingly occurs inside electronic devices. This results in erroneous operation of the electronic device itself and erroneous operation of an external electronic device caused by noise leaking from the device.
In such an electronic device, a great number of components, such as chip capacitors, ferrite beads, EMI filters or the like, are employed as countermeasures against noise. Among these noise blocking components, ferrite chip bead elements are employed as countermeasures against noise in a great number of electronic devices since they are inexpensive and easy to use.
An ideal ferrite chip bead element should be capable of allowing necessary signals to pass while not allowing any unwanted signals (may be referred to as noise) present in the higher frequency range to pass, and it is required to demonstrate signal absorption characteristics over a broad frequency range.
However, since the electromagnetic characteristics of the ferrite material used in ferrite chip bead elements normally demonstrate dependency on the frequency, it is difficult to achieve broad-band signal absorption characteristics. In addition, since, theoretically, a spinel type ferrite bead obtained through sintering does not demonstrate magnetism in the GHz frequency range, they are not suited for use in high frequency ranges.
As a noise blocking component that may be employed in the high frequency ranges, an absorption-type low pass filter has already been proposed. One example of such an absorption-type low pass filter is constituted by using ferrite and is widely employed as a laminated noise blocking component.
This laminated noise blocking component, which achieves a structure realized by embedding a signal conductor inside a magnetic layer constituted of ferrite, is constituted by laminating paste containing ferrite and paste or metal foil containing a conductor by employing printing technology or a thick film sheet lamination method.
Since Ag or an Ag alloy is used to constitute the signal conductor, a well known example of this laminated noise blocking component uses magnetic layers constituted of NiCuZn ferrite or the like that can be sintered at a relatively low temperature.
However, since an NiCuZn ferrite material, which achieves a dielectric constant of approximately 10 to 15, is not suited for use in a high frequency range, since the floating capacity between the patterns of the signal conductor is large and, therefore, the self-resonant frequency cannot be increased.
There are other magnetic materials such as Mn -group ferrites that are materials achieving high magnetic permeability and planar group ferrites suited for use in a high frequency range. However, they all need to be sintered at a high temperature, requiring control of the baking atmosphere and, therefore, they are not suited to be baked at the same time with the metal that is used to constitute the signal conductor.
Furthermore, the impedance peak value of a laminated noise blocking component achieved in the form of a ferrite chip bead element is only 700 to 800 MHz even when a material having a low magnetic permeability (.mu.) is used, proving that it is not suited for noise absorption in the GHz frequency range.
In addition, while the impedance value may be increased by increasing the number of turns in the signal conductor or by using a material having a high magnetic permeability (.mu.), the laminated noise blocking component is still not suited for absorbing noise in the high frequency range, since the impedance peak value shifts toward a lower frequency range.
U.S. Pat. No. 4,297,661 discloses a low pass filter to be employed in a high frequency range, in which the microstrip is constituted of ferrite. This filter is achieved by taking advantage of the phenomenon in which the absorption effect manifests in the low frequency range and disappears in the high frequency range, but is still not capable of inhibiting unwanted signal components present in the high frequency range of GHz or higher through absorption.
While Schiffres proposes a coaxial transmission line employing ferrite in IEEE Transactions on Electromagnetic Compatibility page 55 to 61, 1964, the object of this coaxial transmission line is acquisition of characteristics mainly in the MHz band and the transmission characteristics and the reflection characteristics in the frequency range of GHz or higher are not disclosed. It is assumed that transmission occurs in the transmission line in the high frequency range of GHz or higher.
An attempt to effect signal removal through absorption in the high frequency range by combining a non-magnetic material having an absorption effect in the high frequency range and ferrite, too, has been reported on.
The EMI filter proposed by Schlicke in IEEE Spectrum page 59 to 68, 1967 and the pass EMI filter effective in the low frequency range proposed by Bogar in Proceedings of the IEEE vol. 67 page 159 to 163, 1979 are examples of this attempt. In each of these filters in the prior art, a portion of the insulator in the coaxial filter is constituted by laminating ferrite and a dielectric substance. In U.S. Pat. No. 4,146,854, a blocking element that employs a wave absorber constituted of a composite material of ferrite beads, resin and metal or the like is disclosed, and in Japanese Unexamined Patent Publication No. 127701/1992, a technology that employs a wave absorbing substance in a portion of a non-magnetic microstrip line is disclosed.
However, in either case, the wave absorber or the wave absorbing substance is merely used in an auxiliary manner for the purpose of keeping down the high frequency components of the signal that cannot be absorbed.
Furthermore, U.S. Pat. No. 4,301,428 discloses an electrical wire, a cable or the like containing a metallic magnetism absorbing mixture to function as a conductive element having an appropriate degree of electrical resistance. This conductive element has a compound structure achieved by covering a non conductive core constituted of fiber, resin or glass with a thin conductive metal layer.
However, providing electrical resistance at a signal line would prove problematic in applications in which micro signals are handled, since it would not only remove the noise component but also result in blocking of the signal component. In addition, this prior art publication only discloses an electrical wire and does not mention any functions to be fulfilled as a circuit element.
Japanese Unexamined Patent Publication No. 78218/1996 discloses a chip bead element to support high frequencies that is constituted by providing a signal line electrode inside an insulating body constituted of a composite material achieved by mixing a ferromagnetic metal powder and an insulating resin and providing a ground electrode at a front surface of the insulating body.
While this chip bead element achieves high pass characteristics and low pass characteristics, whereby the high frequency component of 1 GHz or higher can be absorbed, its frequency blocking characteristics in the high frequency range become steep, since the chip bead element uses ferromagnetic metal powder. Thus, it is not suited for increasing the frequency blocking range.
The electromagnetic characteristics of ferrite materials and compound ferrite materials demonstrate frequency dependence, and resonance occurs as the frequency increases. Then, the magnetic permeability (.mu.) becomes drastically reduced when the frequency exceeds a given level. This limits frequencies that can be used.
In order to achieve effectiveness over a broader frequency range, an element achieved by providing beads having varying frequency blocking characteristics in series may be employed.
However, there is a problem with the wound-type line in that the element becomes large. In addition, while the structure achieved by baking circuits constituted of different ferrite materials having different frequency characteristics that have been simultaneously laminated in series may be conceived for a laminated type element, problems such as cracking, peeling and warping occur in the element constituted by laminating two or more different ferrite materials and baking them at the same time, due to the varying coefficients of linear thermal expansion and the varying contraction behavior of the materials.
Normally, when simultaneously baking a laminated body constituted by laminating different materials, if the difference between the coefficients of linear thermal expansion between the two materials reaches or exceeds 15.times.10.sup.-7 /.degree. C., cracks occur inside the laminated chip bead element.
In addition, even when the difference between the coefficients of linear thermal expansion is less than 15.times.10.sup.-7 / .degree. C., it is difficult to achieve electrical and magnetic characteristics as designed, due to stress applied to the ferrite which results in degradation in the magnetic characteristics.
Furthermore, when simultaneously baking different materials, since the compositions of the materials used are different, a reaction occurs at the layer interface, which causes copper oxide or the like to become deposited at the layer interface to greatly reduce the inherent resistance of the element.
Moreover, when simultaneously baking different materials to constitute a laminated chip bead element, only NiCuZn ferrite that can be sintered at a low temperature can be used as the ferrite material. Thus, the operating frequencies become limited and only a narrow band can be supported.